Anticoagulants serve a need in the marketplace in treatment or prevention of undesired thrombosis in patients with a tendency to form blood clots, such as, for example, those patients having clotting disorders, confined to periods of immobility or undergoing medical surgeries. One of the major limitations of anticoagulant therapy, however, is the bleeding risk associated with the treatments, and limitations on the ability to rapidly reverse the anticoagulant activity in case of overdosing or if an urgent surgical procedure is required. Thus, specific and effective antidotes to all forms of anticoagulant therapy are highly desirable. For safety considerations, it is also advantageous to have an anticoagulant-antidote pair in the development of new anticoagulant drugs.
Currently available anticoagulant-antidote pairs for over-anticoagulation are heparin—protamine and warfarin—vitamin K. Fresh frozen plasma and recombinant factor VIIa (rfVIIa) have also been used as non-specific antidotes in patients under low molecular weight heparin treatment, suffering from major trauma or severe hemorrhage. (Lauritzen, B. et al, Blood, 2005, 607A-608A.) Also reported are protamine fragments (U.S. Pat. No. 6,624,141) and small synthetic peptides (U.S. Pat. No. 6,200,955) as heparin or low molecular weight heparin antidotes; and thrombin muteins (U.S. Pat. No. 6,060,300) as antidotes for thrombin inhibitors. Prothrombin intermediates and derivatives have been reported as antidotes to hirudin and synthetic thrombin inhibitors (U.S. Pat. Nos. 5,817,309 and 6,086,871).
One promising form of anticoagulant therapy targets factor Xa (fXa), and in fact, several direct fXa inhibitors are currently in different stages of clinical development for use in anticoagulant therapy. One direct fXa inhibitor Xarelto™ (rivaraoxaban) has been approved for clinical use in the European Union and Canada for the prevention of venous thromboembolism in orthopedic surgery patients. Many of these are small molecules. While these new fXa inhibitors show promise for treatment, specific and effective antidotes are still needed. In cases of over-anticoagulation or requirement for surgery in patients treated with these fXa inhibitors, an agent may be required to substantially neutralize the administered fXa inhibitor or inhibitors and restore normal hemostasis.
Currently available agents, such as recombinant factor VIIa (rfVIIa), are mechanistically limited and not specific for reversal of fXa inhibitors and thus improved options for the clinician are highly desirable. In human studies, rfVIIa has been used to reverse the effect of indirect antithrombin III dependent fXa inhibitors such as fondaparinux and idraparinux (Bijsterveld, N R et al, Circulation, 2002, 106:2550-2554; Bijsterveld, N R et al, British J. of Haematology, 2004(124): 653-658). The mechanism of action of factor VIIa (fVIIa) is to act with tissue factor to convert factor X (fX) present in blood circulation to fXa to restore normal hemostasis in patients. This mode of action necessarily dictates that the highest potential concentration of fXa that could be attained to neutralize active site directed fXa inhibitors is limited by the circulating plasma concentration of fX. Since the circulating plasma concentration of fX is 150 nanomolar (“nM”), the maximal amount of fXa produced by this mode would be 150 nM. Thus, the potential of using rfVIIa to reverse the effect of direct fXa inhibitors is mechanistically limited. Reported therapeutic concentrations of small molecule fXa inhibitors such as rivaroxaban have been higher (approximately 600 nM, Kubitza D, et al, Eur. J. Clin. Pharmacol., 2005, 61:873-880) than the potential amount of fXa generated by rfVIIa. Use of rfVIIa for reversal of therapeutic or supratherapeutic levels of anticoagulation by fXa inhibitor would therefore provide inadequate levels of efficacy. As shown in FIG. 4, using rfVIIa has limited effect in neutralizing the anticoagulant activity of a factor Xa inhibitor betrixaban (described below). Recombinant fVIIa showed a dose responsive antidote activity from 50 nM to 100 nM, but the effect leveled off between 100 nM to 200 nM, indicating that its antidote effect is limited by factors other than its concentration. In all of the rfVIIa concentrations tested, betrixaban still showed a dose responsive inhibition of fXa, up to about 75% inhibition at a concentration of 250 nM. This observation is consistent with fVIIa's proposed mechanism of action. This is also supported by studies showing that rfVIIa did not completely reverse the inhibitory effect of fondaparinux on the parameters of thrombin generation and prothrombin activation. (Gerotiafas, G T, et al, Thrombosis & Haemostasis 2204(91):531-537).
Exogenous active fXa cannot be administered directly to a subject in a way similar to rfVIIa. Unlike rfVIIa, which has very low procoagulant activity in the absence of its cofactor tissue factor, native fXa is a potent enzyme and has a potential risk of causing thrombosis. Thus, the use of either rfVIIa or active fXa as an antidote to a fXa anticoagulant therapy has disadvantages.
Thus, there is a need for improved antidote agents that do not cause undesired thrombosis and that are effective in substantially neutralizing the anticoagulant activity of a fXa inhibitor in the event of an overdose of the fXa inhibitor or in the event that normal hemostasis needs to be restored to prevent or stop bleeding.
Any and all publications, patents, patent applications mentioned herein are hereby incorporated by reference in their entirety.